1. Field of the Invention
The present invention relates to a multiple-input multiple-output (MIMO) system and method in a wireless communication environment, and in one embodiment to a MIMO method and system that facilitates backwards compatibility with legacy devices.
2. Description of the Related Art
The design of communication systems for wireless local area networks (WLANs) is based on a family of standards described in IEEE 802.11. For example, the 802.11a specification provides up to 54 Mbps in the 5 GHz band whereas the 802.11g specification also provides up to 54 Mbps but in the 2.4 GHz band. Both the 802.11a/g specifications use an orthogonal frequency division multiplexing (OFDM) encoding scheme.
Notably, the 802.11a/g specifications provide for only one data stream being transmitted or received at any given time. For example, FIG. 1 illustrates a simplified system 100 including a transmitter 101 that can provide a single output at any given time and a receiver 102 that can process a single input at any given time. Thus, system 100 is characterized as a single input single output system.
To address multipath and, more particularly, the fading caused by multipath (wherein objects in the environment can reflect a transmitted wireless signal) and other conditions, a wireless system can employ various techniques. One such technique is switch diversity, wherein transmitters and/or receivers can selectively switch between multiple antennas. For example, FIG. 2 illustrates a simplified system 200 in which transmitter 101 can choose to send signals from antenna 201A or antenna 201B (using a switch 203) whereas receiver 102 can choose to process signals from antenna 202A or antenna 202B (using a switch 204). Thus, system 200 is characterized as a switched diversity antenna configuration.
FIG. 3 illustrates a simplified multiple-input multiple-output (MIMO) system 300, which can transmit on multiple antennas simultaneously and receive on multiple antennas simultaneously. Specifically, a transmitter 301 can transmit signals simultaneously from antenna 302A (using a transmitter chain 303A) and from antenna 302B (using a transmitter chain 303B). Similarly, a receiver 304 can receive signals simultaneously from antenna 305A (using a receiver chain 306A) and from antenna 305B (using a receiver chain 306B).
Note that there are a number of types of MIMO systems. For example, MIMO-AG refers to a MIMO system compatible with both 802.11a and 802.11g. In contrast, MIMO-SM refers to a MIMO system with spatial multiplexing. The use of the acronym “MIMO” hereinafter refers to MIMO-SM.
The use of multiple antennas, depending on the specific implementation, can either extend the range or increase the data rate at a given range. For example, FIG. 4 illustrates the median data rates for various antenna configurations over relative distances. Waveform 401 represents a single antenna configuration; waveform 402 represents a switched diversity antenna configuration; and waveform 403 represents a MIMO antenna configuration. Notably, at any relative distance between 2 and 4, the median data rate for the MIMO antenna configuration is significantly greater than the median data rates for either the single antenna configuration or the switched diversity antenna configuration. For example, at relative distance 3, which represents a top end for a typical home space 404, the median data rate for a MIMO antenna configuration (50 Mbps) is significantly greater than the median data rates for a single antenna configuration (18 Mbps) or even for a switched diversity antenna configuration (33 Mbps).
A MIMO system can also advantageously minimize the differences in signal to noise ratio (SNR) across different frequency bins. For example, FIG. 5 illustrates the SNRs for various antennas across various frequency bins, i.e. SNRs 501 for a first antenna (waveform represented by the dotted line), SNRs 502 for a second antenna (waveform represented by the dashed line), and SNRs 503 for simultaneous usage of the first and second antennas (waveform represented by the solid line). Note that both SNRs 501 and 502 can vary significantly over frequency bins 0-60. In contrast, a MIMO system simultaneously using both the first and second antennas, shown by SNRs 503, can minimize the differences in SNR across different frequency bins (i.e. notches on one channel are compensated for by non-notches in the other channel), thereby allowing more effective compensation for such SNR in the receiver chains and/or transmitter chains.
In MIMO system 300 (FIG. 3), receiver 304 uses multiple chains (i.e. chains 306A and 306B) to receive and decode the multiple data streams (e.g. packets) transmitted by transmitter 301. Unfortunately, because a legacy 802.11a/g device is not able to decode multiple data streams, such a legacy device may “stomp” on a MIMO packet by transmitting before the transmission of the MIMO packet is complete.
Therefore, a need arises for a MIMO system and method that allows legacy devices to decode the length of a MIMO packet and to restrain from transmitting during that period. A further need arises for an efficient way to transmit MIMO packets.